In order to secure the safe driving of automobiles, various tire air pressure warning system (Tire Pressure Monitoring System; TPMS) have been conventionally suggested according to which a decreased air pressure of a tire attached to the vehicle is detected to warn the driver for an appropriate treatment.
A conventional TPMS can be classified into two types of the direct TPMS and the indirect TPMS. The direct TPMS provides a pressure sensor in a tire to thereby directly measure the tire pressure. The direct TPMS can detect a decrease in the pressure at a high accuracy but requires exclusive wheels and is involved with some disadvantages in the markets, thus leaving technical and cost disadvantages.
The indirect TPMS is a method of estimating the air pressure based on the tire rotation information. The indirect TPMS can be further classified into the Dynamic Loaded Radius (DLR) method and the Resonance Frequency Method (RFM).
The DLR method is a method that uses a phenomenon according to which deflated tires is collapsed and thus the tire has a reduced dynamic loaded radius and is consequently rotated at a higher speed than other tires with a normal pressure. The DLR method compares the rotation speeds of the four tires to thereby detect deflated tires (see Patent Literature 1 for example).
The RFM method on the other hand is a method to use a fact that deflated tires has a different wheel speed signal frequency characteristic to thereby detect a difference from a normal pressure. In contrast with the DLR method, the RFM method is an absolute comparison with the normal values of the respective wheels that are retained in advance. Thus, the RFM method also can detect the four tires simultaneous deflation condition. Thus, the RFM method attracts attentions as a better indirect detection method (see Patent Literature 2 for example). In case of the RFM method, attention is paid on the resonance frequency obtained by the frequency analysis of wheel speed signals. Then, warning is issued when it is determined that the resonance frequency is relatively lower than a reference frequency assumed during the calibration.
By the way, the current situation is that the regulation-compatible indirect TPMS can detect a deflated tire and three tires with a normal pressure, two deflated tires (decreased pressures of the same level) and two tires with normal pressure, three deflated tires (decreased pressures of the same level) and a tire with a normal pressure, and four deflated tires (decreased pressures of the same level). The detection level is set so that, in consideration of the convenience in the market, no warning is issued at a 10%-deflation condition for example and requirements specified by regulations (North America: 25%-deflation condition, Europe: 20%-deflation condition) are satisfied.
Among the above-described various deflation conditions, one deflated tire and three tires with a normal pressure, two diagonal deflated tires and other two tires with a normal pressure, and three deflated tires and one tire with a normal pressure can be detected by comparison of DLRs of the two diagonal tires to each other. Two rear axle deflated tires (decreased pressures of the same level) and two front axle tires with a normal pressure can be detected by comparison of the DLRs of two coaxial tires to each other. The same side deflated tires (right side or left side) (decreased pressures of the same level) and two tires with a normal pressure can be detected by comparison of the DLRs of the two tires of the same side to each other. Two front axle deflated tires (decreased pressures of the same level) and two rear axle tires with a normal pressure four deflated tires (decreased pressures of the same level) can be detected by the RFM method.